Polymers exhibiting very high melting points or very high viscosities above their melting points often times have very desirable properties such as chemical resistance, temperature resistance, high strength and a plethora of other characteristics needed for very high performance. A great difficulty, however, is that these polymers tend to be intractable, that is, not melt processable, wherein forming into shaped articles is expensive at best and impossible in many cases.
For example, nylons of hexamethylene diamine and terephthalic acid exhibit excellent temperature resistance but cannot be melt-spun because they decompose before their crystalline melting temperature is reached. Likewise, many other wholly aromatic polymers such as polyimides of pyromellitic anyhydride and aromatic diamines cannot be melt-processed in polyamic acid or fully imidized form.
One way of overcoming processing problems of high performance polymers, especially in connection with polybenzimidazoles is disclosed in U.S. Pat. No. 4,814,530 to Ward et al. The '530 patent describes a process for making articles by compacting melt-derived polybenzimidazole particulate resin under a pressure of about 2,000 to 10,000 psi while heating the constrained object to a temperature of 825.degree. to 950.degree. F. Melt-derived polybenzimidazole powder is produced by polymerizing an aromatic tetraamine and a diphenylester or an anhydride of an aromatic or heterocyclic dicarboxylic acid in a one or two stage process. In the two stage process, the monomers are heated at a temperature above about 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The formed prepolymer is cooled, pulverized and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product.
Commonly assigned U.S. Pat. No. 4,628,067, to Chen et al., incorporated herein by reference, discloses microporous polybenzimidazole particulates having a reduced bulk density, a surface area of at least 10 m.sup.2 /gm, a porosity of at least about 10%, and an average pore diameter ranging from about 50 to 500 Angstroms produced by the method of emulsification-precipitation or atomizing non-solvent quenching techniques. The method of production involves dissolving the polybenzimidazole resin in a suitable solvent to produce a polybenzimidazole solution followed by emulsification-precipitation or atomizing the polybenzimidazole solution onto a static surface of methanol to form the polybenzimidazole particulates.